1. Field of the Invention
The present invention relates generally to improved methods of displacing liquids through pipes, and more particularly, to improved methods of cementing pipe, such as casing, in a well bore.
2. Description of the Prior Art
A fluid of a particular type, composition, viscosity and/or other physical properties is frequently displaced through a pipe by a second fluid of a different type, composition, viscosity or other property. For example, underground pipelines for conducting fluids, such as hydrocarbons, e.g., oil or gas, aqueous solutions, aqueous dispersions and the like from one location to another, are commonly utilized throughout the world. Very often, it is necessary to displace a first fluid through a pipeline by a second fluid without mixing the two fluids. This has heretofore been accomplished by physically inserting a resilient plug or ball between the two fluids. The plug functions to separate the fluids, prevent them from being mixed and also to wipe the walls of the pipe and remove residue therefrom as the first fluid is displaced through the pipe by the second fluid.
Fluid-separating plugs are frequently utilized in pipelines and in the construction and repair of oil and gas wells. For example, primary cementing, a principal well construction technique, involves the placement of a hydraulic cement slurry in the annulus between the walls of a subterranean well bore and a pipe, e.g., casing, disposed therein. The cement slurry is allowed to set in the annulus whereby the exterior surfaces of the pipe are bonded to the walls of the well bore. This bonding of the pipe within the well bore serves to maintain the pipe in place and to prevent formation fluids from communicating between subterranean formations or zones or to the surface by way of the annulus.
The performance of a primary cementing operation is generally conducted after a well bore has been drilled to the depth to which casing is to be set. The pipe to be cemented in the well bore is suspended in the well bore which is filled with liquid drilling fluid used in the drilling operation, whereby both the pipe and the well bore contain liquid drilling fluid. A hydraulic cement slurry is pumped downwardly within the pipe whereby the cement slurry displaces the drilling fluid within the pipe into the annulus and drilling fluid in the annulus out of the well bore at the surface. When the required volume of cement slurry has been pumped into the pipe, a displacement fluid such as an aqueous solution is utilized to displace the cement slurry into the annulus. That is, the displacement fluid is pumped downwardly within the interior of the pipe in a volume such that the cement slurry in the pipe is displaced into the annulus.
In order to prevent the cement slurry used in primary cementing from mixing with and being contaminated by drilling fluid and the displacement fluid utilized, resilient separating plugs have heretofore been utilized. That is, the bottom of the pipe to be cemented in the well bore usually has a flow control device, such as a float shoe, connected thereto which contains a check valve for allowing fluids to flow out of the pipe and into the annulus but prevents backflow from the annulus into the pipe. Such float shoes are well known to those skilled in the art and they generally include an upwardly facing seating surface. A resilient plug, known as a bottom plug, formed of a rubber or plastic material is inserted at the top of the pipe between the drilling fluid contained therein and the cement slurry to be pumped into the annulus. The cement slurry is pumped into the pipe which displaces the bottom plug and the drilling fluid downwardly through the pipe and through the check valve in the float shoe until the plug contacts the seating surface of the float shoe. The plug functions to separate the cement slurry from the drilling fluid and to wipe drilling fluid from the walls of the pipe as it is displaced therethrough. While various techniques have been utilized for allowing the cement slurry to flow through or around the bottom plug when the cement slurry is displaced into the annulus, a common technique is to include a rupturable member across a passageway extending through the plug whereby when the plug is seated on the float shoe and the pressure against the plug is increased, the rupturable member ruptures which allows the cement slurry to flow through the passageway in the plug, through the float shoe and into the annulus.
When the required quantity of cement slurry is pumped into the pipe the pumping of the cement slurry is stopped and a second plug, known as a top plug, is placed between the cement slurry and a displacement fluid to be pumped into the pipe. The displacement fluid is next pumped into the pipe whereby the top plug and the cement slurry are displaced downwardly and the cement slurry is displaced into the annulus. The displacement fluid is pumped until the top plug above the cement slurry contacts the bottom plug seated on the float shoe. When that happens, the required volume of cement slurry has been displaced into the annulus and the cement slurry is allowed to set therein. After the cement slurry has set, the plugs and the float shoe are drilled out, and further drilling and well completion operations are commenced.
Heretofore, the physical placement of the resilient plugs in the pipe has been accomplished using a plug container which is connected to the pipe and includes relatively complex plug releasing devices. Such containers are difficult to install and load, and in performing primary well cementing operations using plugs, a plug container operator is often required to work above the drilling rig floor suspended from a line connected to the drilling rig. In addition, since a positive indication of the release of a plug is required, pumping of the cement slurry or displacement fluid must often be interrupted for a relatively long period of time while the plug is released or while an additional plug is loaded into the plug container. Thus, there is a need for an improved method of providing fluid separating plugs within pipes in well cementing and similar operations. Also, as mentioned above, in the operation of pipelines fluid separation and pipeline cleaning is accomplished by inserting a plug, sometimes referred to as a pig, in the pipeline. The placement of such a plug has heretofore required the use of special plug containment loops, valves and the like. Thus, there is also a need for improved methods of providing plugs in pipelines and other similar operations.